Three-way catalyst and its use in exhaust systems

ABSTRACT

A three-way catalyst is disclosed. The three-way catalyst comprises a silver-containing extruded zeolite substrate and a catalyst layer disposed on the silver-containing extruded zeolite substrate. The catalyst layer comprises a supported platinum group metal catalyst comprising one or more platinum group metals and one or more inorganic oxide carriers. The invention also includes an exhaust system comprising the three-way catalyst. The three-way catalyst results in improved hydrocarbon storage and conversion, in particular during the cold start period.

FIELD OF THE INVENTION

The invention relates to a three-way catalyst and its use in an exhaustsystem for internal combustion engines.

BACKGROUND OF THE INVENTION

Internal combustion engines produce exhaust gases containing a varietyof pollutants, including hydrocarbons, carbon monoxide, and nitrogenoxides (“NO_(x)”). Emission control systems, including exhaust gascatalysts, are widely utilized to reduce the amount of these pollutantsemitted to atmosphere. A commonly used catalyst for gasoline engineapplications is the “three-way catalyst” (TWC). TWCs perform three mainfunctions: (1) oxidation of CO; (2) oxidation of unburnt hydrocarbons;and (3) reduction of NO_(x) to N₂.

TWCs, like other exhaust gas catalysts, typically achieve very highefficiencies once they reach their operating temperature (typically,200° C. and higher). However, these systems are relatively inefficientbelow their operating temperature (the “cold start” period). As evenmore stringent national and regional legislation lowers the amount ofpollutants that can be emitted from diesel or gasoline engines, reducingemissions during the cold start period is becoming a major challenge.Thus, methods for reducing the level of NO_(x) and hydrocarbons emittedduring cold start condition continue to be explored.

For cold start hydrocarbon control, hydrocarbon (HC) traps includingzeolites as hydrocarbon trapping components have been investigated. Inthese systems, the zeolite component adsorbs and stores hydrocarbonsduring the start-up period and releases the stored hydrocarbons when theexhaust temperature is high enough to desorb hydrocarbons. The desorbedhydrocarbons are subsequently converted by a TWC component eitherincorporated into the HC trap or by a separate TWC placed downstream ofthe HC trap.

For instance, U.S. Pat. No. 5,772,972 discloses a hybrid system ofhydrocarbon trapping material and palladium based three-way catalystmaterial. U.S. Pat. No. 6,617,276 teaches a catalyst structurecomprising a first layer consisting essentially of ahydrocarbon-adsorbing zeolite and a K, Rb, or Cs active metal that isimpregnated on the zeolite, at least one additional layer consistingessentially of at least one platinum group metal, and a catalystsubstrate on which the first layer and the one or more additional layersare disposed. In EP 1129774, a hydrocarbon adsorbing member is taughtthat comprises a zeolite having SiO₂:Al₂O₃ molar ratio of 100 or moreand an average primary particle diameter of 1 μm or less of, and that itis free from a monovalent metallic element. U.S. Pat. No. 6,074,973teaches a catalyzed hydrocarbon trap material comprising palladium andsilver dispersed on a high surface area metal oxide support and azeolite material such as one or more of ZSM-5, Beta, Y, and othersuitable zeolites.

U.S. Appl. Pub. No. 2012/0117953 A1 teaches a three way catalyst thatcomprises an extruded solid body comprising 10-100 weight percent of atleast one binder/matrix component, 5-90 weight percent of a zeoliticmolecular sieve, a non-zeolitic molecular sieve or a mixture of any twoor more thereof, and 0-80 weight percent of an optional stabilizedceria. The catalyst comprises at least one precious metal and optionallyat least one non-precious metal, wherein: (i) the at least one preciousmetal is carried in one or more coating layer(s) on a surface of theextruded solid body; (ii) at least one metal is present throughout theextruded solid body and at least one precious metal is also carried inone or more coating layer(s) on a surface of the extruded solid body; or(iii) at least one metal is present throughout the extruded solid body,is present in a higher concentration at a surface of the extruded solidbody and at least one precious metal is also carried in one or morecoating layer(s) on the surface of the extruded solid body. In addition,U.S. Appl. Pub. No. 2012/0308439 A1 teaches a cold start catalyst thatcomprises (1) a zeolite catalyst comprising a base metal, a noble metal,and a zeolite, and (2) a supported platinum group metal catalystcomprising one or more platinum group metals and one or more inorganicoxide carriers.

As with any automotive system and process, it is desirable to attainstill further improvements in exhaust gas treatment systems,particularly under cold start conditions. We have discovered a newthree-way catalyst that provides enhanced cleaning of the exhaust gasesfrom internal combustion engines.

SUMMARY OF THE INVENTION

The invention is a three-way catalyst for use in an exhaust system. Thethree-way catalyst comprises a silver-containing extruded zeolitesubstrate. The three-way catalyst also comprises a catalyst layer thatis disposed on the silver-containing extruded zeolite substrate. Thecatalyst layer comprises a supported platinum group metal catalystcomprising one or more platinum group metals and one or more inorganicoxide carriers. The invention also includes an exhaust system comprisingthe three-way catalyst.

DETAILED DESCRIPTION OF THE INVENTION

The three-way catalyst of the invention comprises a silver-containingextruded zeolite substrate.

The zeolite of the silver-containing extruded zeolite substrate may beany natural or a synthetic zeolite, including molecular sieves, and ispreferably composed of aluminum, silicon, and/or phosphorus. Thezeolites typically have a three-dimensional arrangement of SiO₄, AlO₄,and/or PO₄ that are joined by the sharing of oxygen atoms. The zeoliteframeworks are typically anionic, which are counterbalanced by chargecompensating cations, typically alkali and alkaline earth elements(e.g., Na, K, Mg, Ca, Sr, and Ba) and also protons.

The zeolite is preferably a beta zeolite, a faujasite (such as anX-zeolite or a Y-zeolite, including NaY and USY), an L-zeolite, a ZSMzeolite (e.g., ZSM-5, ZSM-48), an SSZ-zeolite (e.g., SSZ-13, SSZ-41,SSZ-33), an AEI framework zeolite, a mordenite, a chabazite, anoffretite, an erionite, a clinoptilolite, a silicalite, an aluminumphosphate zeolite (including metalloaluminophosphates such as SAPO-34),a mesoporous zeolite (e.g., MCM-41, MCM-49, SBA-15), or mixturesthereof; more preferably, the zeolites are beta zeolite, ZSM-5 zeolite,or SSZ-33, or Y-zeolite. The zeolite is most preferably beta zeolite,ZSM-5 zeolite, or SSZ-33.

The extruded zeolite substrate may be formed by any known means.Typically, the zeolite is extruded to form a flow-through or filtersubstrate, and is preferably extruded to form a honeycomb flow-throughmonolith. Extruded zeolite substrates and honeycomb bodies, andprocesses for making them, are known in the art. See, for example, U.S.Pat. Nos. 5,492,883, 5,565,394, and 5,633,217 and U.S. Pat. No. Re.34,804. Typically, the zeolite material is mixed with a permanent bindersuch as silicone resin and a temporary binder such as methylcellulose,and the mixture is extruded to form a green honeycomb body, which isthen calcined and sintered to form the final extruded zeolite substrate.

The extruded zeolite substrate may be formed as a flow-through or filtersubstrate. If formed as a flow-through substrate, it is preferably aflow-through monolith having a honeycomb structure with many small,parallel thin-walled channels running axially through the substrate andextending throughout from an inlet or an outlet of the substrate. Thechannel cross-section of the substrate may be any shape, but ispreferably square, sinusoidal, triangular, rectangular, hexagonal,trapezoidal, circular, or oval.

If formed as a filter substrate, the silver-containing extruded zeolitesubstrate is preferably a wall-flow monolith filter. The channels of awall-flow filter are alternately blocked, which allow the exhaust gasstream to enter a channel from the inlet, then flow through the channelwalls, and exit the filter from a different channel leading to theoutlet. Particulates in the exhaust gas stream are thus trapped in thefilter.

The zeolite may contain silver prior to extruding such that thesilver-containing extruded zeolite substrate is produced by theextrusion procedure. If the zeolite contains silver prior to extrusion,the silver may be added to the zeolite to form a silver-containingzeolite by any known means, the manner of addition is not considered tobe particularly critical. For example, a silver compound (such as silvernitrate) may be added to the zeolite by impregnation, adsorption,ion-exchange, incipient wetness, precipitation, or the like.

If an extruded zeolite substrate is first formed without silver, theextruded zeolite monolith is then loaded with silver to produce thesilver-containing extruded zeolite substrate. Preferably, the extrudedzeolite monolith is subjected to an impregnation procedure to loadsilver onto the zeolite monolith.

Preferably, the silver-containing extruded zeolite substrate comprisesfrom 1 to 700 g/ft³ silver, more preferably from 10 to 200 g/ft³ silver.

The three-way catalyst also comprises a catalyst layer that is disposedon the silver-containing extruded zeolite substrate. The catalyst layercomprises a supported platinum group metal catalyst. The supportedplatinum group metal catalyst comprises one or more platinum groupmetals (“PGM”) and one or more inorganic oxide carriers. The PGM may beplatinum, palladium, rhodium, or combinations thereof, and mostpreferably palladium and rhodium. The inorganic oxide carriers mostcommonly include oxides of Groups 2, 3, 4, 5, 13 and 14 elements. Usefulinorganic oxide carriers preferably have surface areas in the range 10to 700 m²/g, pore volumes in the range 0.1 to 4 mL/g, and pore diametersfrom about 10 to 1000 Angstroms. The inorganic oxide carrier ispreferably alumina, silica, titania, zirconia, ceria, niobia, tantalumoxides, molybdenum oxides, tungsten oxides, or mixed oxides or compositeoxides of any two or more thereof, e.g. silica-alumina, ceria-zirconiaor alumina-ceria-zirconia. Alumina and ceria-zirconia are particularlypreferred.

The supported platinum group metal catalyst may be prepared by any knownmeans. Preferably, the one or more platinum group metals are loaded ontothe one or more inorganic oxides by any known means to form thesupported PGM catalyst, the manner of addition is not considered to beparticularly critical. For example, a palladium compound (such aspalladium nitrate) may be supported on an inorganic oxide byimpregnation, adsorption, ion-exchange, incipient wetness,precipitation, or the like. Other metals may also be added to thesupported PGM catalyst.

The supported PGM catalyst layer is disposed on the silver-containingextruded zeolite substrate. The supported PGM catalyst layer may bedisposed on the silver-containing extruded zeolite substrate byprocesses well known in the prior art. Preferably, the supportedplatinum group metal catalyst is coated onto the silver-containingextruded zeolite substrate using a washcoat procedure to produce athree-way catalyst of the invention.

A representative process for preparing the three-way catalyst using awashcoat procedure is set forth below. It will be understood that theprocess below can be varied according to different embodiments of theinvention.

The washcoating procedure is preferably performed by first slurryingfinely divided particles of the supported platinum group catalyst in anappropriate solvent, preferably water, to form the slurry. Additionalcomponents, such as transition metal oxides, binders, stabilizers, orpromoters may also be incorporated in the slurry as a mixture of watersoluble or water-dispersible compounds. The slurry preferably containsbetween 10 to 70 weight percent solids, more preferably between 30 to 50weight percent. Prior to forming the slurry, the supported platinumgroup catalyst particles are preferably subject to a size reductiontreatment (e.g., milling) such that the average particle size of thesolid particles is less than 20 microns in diameter.

The silver-containing extruded zeolite substrate may then be dipped oneor more times into the slurry or the slurry may be coated on thesubstrate such that there will be deposited on the silver-containingextruded zeolite substrate the desired loading of catalytic materials.Alternatively, a slurry containing only the inorganic oxide(s) may firstbe deposited on the zeolite catalyst-coated substrate to form aninorganic oxide-coated substrate, followed by drying and calcinationsteps. The platinum group metal(s) may then be added to the inorganicoxide-coated substrate by any known means, including impregnation,adsorption, or ion-exchange of a platinum group metal compound (such asplatinum nitrate).

Preferably, the entire length of the silver-containing extruded zeolitesubstrate is coated with the slurry so that a washcoat of the supportedplatinum group catalyst covers the entire surface of the substrate.

After the silver-containing extruded zeolite substrate has been coatedwith the supported platinum group catalyst slurry, the coated substrateis preferably dried and then calcined by heating at an elevatedtemperature to form the three-way catalyst. Preferably, the calcinationoccurs at 400 to 600° C. for approximately 1 to 8 hours.

The invention also includes an exhaust system for internal combustionengines comprising the three-way catalyst. The exhaust system preferablycomprises one or more additional after-treatment devices capable ofremoving pollutants from internal combustion engine exhaust gases.

Preferably, the exhaust system comprises a close-coupled catalyst andthe three-way catalyst of the invention. The close-coupled catalyst islocated upstream of the three-way catalyst. Preferably, a particulatefilter may also be added to this system. The particulate filter may belocated downstream of the close-coupled catalyst and upstream three-waycatalyst, or the particular filter may be located downstream of thethree-way catalyst.

Close-coupled catalysts are well known in the art. Close-coupledcatalysts are typically utilized to reduce hydrocarbon emissions duringcold start period following the start of the engine when thetemperature, as measured at the three-way catalyst, will be below atemperature ranging from about 150 to 220° C. Close-coupled catalystsare located within the engine compartment, typically adjacent to theexhaust manifold and beneath the hood, so that they are exposed to hightemperature exhaust gas immediately exiting the engine after the enginehas warmed up.

The close-coupled catalyst preferably comprises a substrate structurecoated with a catalyst layer of a heat-resistant inorganic oxidecontaining at least one noble metal selected from Pt, Pd and Rh. Theheat-resistant substrate is typically a monolith substrate, andpreferably a ceramic substrate or metallic substrate. The ceramicsubstrate may be made of any suitable heat-resistant refractorymaterial, e.g., alumina, silica, titania, ceria, zirconia, magnesia,zeolites, silicon nitride, silicon carbide, zirconium silicates,magnesium silicates, aluminosilicates and metallo aluminosilicates (suchas cordierite and spodumene), or a mixture or mixed oxide of any two ormore thereof. The metallic substrate may be made of any suitable metal,and in particular heat-resistant metals and metal alloys such astitanium and stainless steel as well as ferritic alloys containing iron,nickel, chromium, and/or aluminum in addition to other trace metals(typically, rare earth metals).

The substrate is preferably a flow-through substrate, but may also be afilter substrate. The flow-through substrates preferably have ahoneycomb structure with many small, parallel thin-walled channelsrunning axially through the substrate and extending throughout thesubstrate. If the substrate is a filter substrate, it is preferably awall-flow monolith filter. The channels of a wall-flow filter arealternately blocked, which allow an exhaust gas stream to enter achannel from the inlet, and then flow through the channel walls, andexit the filter from a different channel leading to the outlet.Particulates in the exhaust gas stream are thus trapped in the filter.

The catalyst layer of the close-coupled catalyst is typically added tothe substrate as a washcoat that preferably comprises one or moreinorganic oxides and one or more platinum group metals. The inorganicoxide most commonly includes oxides of Groups 2, 3, 4, 5, 13 and 14elements. Useful inorganic oxides preferably have surface areas in therange 10 to 700 m²/g, pore volumes in the range 0.1 to 4 mL/g, and porediameters from about 10 to 1000 Angstroms. The inorganic oxide ispreferably alumina, silica, titania, zirconia, niobia, tantalum oxides,molybdenum oxides, tungsten oxides, rare earth oxides (in particularceria or neodymium oxide), or mixed oxides or composite oxides of anytwo or more thereof, e.g. silica-alumina, ceria-zirconia oralumina-ceria-zirconia, and can also be a zeolite. The PGMs comprise oneor more of platinum, palladium, and rhodium. The close-coupled catalystmay contain other metals or metal oxides as well.

In another useful embodiment, the exhaust system may also preferablycomprise a conventional oxidation catalyst component and the three-waycatalyst of the invention. In this configuration, the exhaust systemwill preferably contain valves or other gas-directing means such thatduring the cold-start period (below a temperature ranging from about 150to 220° C., as measured at the three-way catalyst, the exhaust gas isdirected to contact the three-way catalyst before flowing to theconventional oxidation catalyst component. Once the after-treatmentdevice(s) reaches the operating temperature (about 150 to 220° C., asmeasured at the three-way catalyst), the exhaust gas flow is thenredirected to contact the conventional oxidation catalyst componentprior to contacting the three-way catalyst. A particulate filter mayalso be added to this by-pass system. U.S. Pat. No. 5,656,244, theteachings of which are incorporated herein by reference, for example,teaches means for controlling the flow of the exhaust gas duringcold-start and normal operating conditions.

The conventional oxidation catalyst component is preferably aconventional TWC catalyst that comprises a substrate coated with a TWClayer. The substrate is typically a monolith substrate, and preferably aceramic substrate or metallic substrate, and is preferably aflow-through substrate but may also be a filter substrate. The TWCcatalyst layer preferably comprises a supported platinum group metalcatalyst that comprises one or more platinum group metals (“PGM”) andone or more inorganic oxide carriers, as described above.

The following examples merely illustrate the invention. Those skilled inthe art will recognize many variations that are within the spirit of theinvention and scope of the claims.

EXAMPLE 1 Preparation of Catalysts of the Invention

Catalyst 1A: Ag on extruded beta zeolite+Pd—Rh layer

A beta zeolite monolith (formed by extruding beta zeolite into ahoneycomb monolith, and containing 55% beta zeolite; see, e.g., U.S.Pat. Nos. 5,492,883, 5,565,394, and 5,633,217) is impregnated with anaqueous silver nitrate solution, followed by drying, and calcining byheating at 500° C. for 4 hours to achieve a Ag loading of 150 g/ft³.

The Ag/beta zeolite monolith is then coated with a typical TWC layerconsisting of alumina and ceria-zirconia mixed oxides as the supportsfor Pd and Rh. The Pd loading is 76.5 g/ft³ and the Rh loading is 8.5g/ft³. The supported coated substrate is dried, and then calcined byheating at 500° C. for 4 hours.

EXAMPLE 2 Comparative Catalyst Preparation

Comparative Catalyst 2A: Extruded beta zeolite+Pd—Rh layer

Comparative Catalyst 2A is prepared according to the same procedure asCatalyst 1A with the exception that the extruded beta zeolite is notimpregnated with silver.

Comparative Catalyst 2B: Cordierite substrate+Pd—Rh layer

Comparative Catalyst 2B is prepared according to the same procedure asCatalyst 1A with the exception that a cordierite substrate monolith isused in place of the Ag/beta zeolite monolith.

EXAMPLE 3 Laboratory Testing Procedures and Results

All the catalysts are tested on core samples (2.54 cm×8.4 cm) of theflow-through catalyst-coated substrate. Fresh catalyst and aged catalystare both tested. Catalyst cores are aged under flow-through conditionsin a furnace under hydrothermal conditions (5% H₂O, balance air) at 800°C. for 80 hours. The cores are then tested for hydrocarbon adsorption ina laboratory reactor, using a feed gas stream that is prepared byadjusting the mass flow of the individual exhaust gas components. Thegas flow rate is maintained at 21.2 L min⁻¹ resulting in a Gas HourlySpace Velocity of 30,000 h⁻¹ (GHSV=30,000 h⁻¹).

The catalysts are pretreated at 650° C. in a gas flow of 2250 ppm O₂,10% CO₂, 10% H₂O, and the balance nitrogen, before cooling to roomtemperature. Following the pretreatment, the catalyst undergoes a HCadsorption step in which the catalyst is contacted for 30 seconds with aHC-containing gas consisting of 1500 ppm (C₁ basis) HC (17 vol. %toluene, 24 vol. % isopentane, and 59 vol. % propene), 1000 ppm NO, 1000ppm CO, 2250 ppm O₂, 10% H₂O, 10% CO₂ and the balance nitrogen. The HCadsorption step is then followed by a HC desorption period in which thecatalyst is subjected to a desorption gas consisting of 200 ppm (C₁basis) HC, 300 ppm O₂, 10% H₂O, 10% CO₂ and the balance nitrogen.

The results on the fresh and aged catalysts for the HC emissions duringthe adsorption period and oxidation period, as well as total HC removed,are shown in Table 1.

EXAMPLE 4 Engine Testing Procedures

Full-sized catalysts of Example 1A and Comparative Examples 2A and 2Bare evaluated on a 2.4 L gasoline vehicle. In each of the tests, acommercial Pd—Rh TWC catalyst is placed in the close-coupled positionupstream of the Example catalysts. The CCC TWC catalyst contains 405g/ft³ Pd and 15 g/ft³ Rh in the front zone, and 105 g/ft³ Pd and 15g/ft³ Rh in the rear zone. Each of the systems is aged for 150 hourswith the Example catalyst bed temperature peaking at 800° C. Thetailpipe total HC (THC) emissions of the systems under FTP 75 testingprotocol are shown in Table 2. Catalyst 1A removes additional 4 mg/mileTHC as compared to the two Comparative Examples.

The results are shown in Table 2.

TABLE 1 NO_(x) Storage Capacity Results % HC adsorbed % HC oxidizedTotal % HC Catalyst (30 sec; 80° C.) (80-650° C.) removed 1A Fresh 93.178.1 81.3 Aged 90.4 62.6 68.8 2A* Fresh 83.5 70.4 73.4 Aged 71.7 61.763.9 2B* Fresh 7.8 69.8 55.9 Aged 9.2 65.4 53.3 *Comparative Example

TABLE 2 Engine Testing Results showing Total Hydrocarbon (THC) EmissionCumulative Tailpipe THC Catalyst (mg/mile) 1A 10 2A * 14 2B * 14 *Comparative Example

We claim:
 1. A three-way catalyst comprising: (1) a silver-containingextruded zeolite substrate; and (2) a catalyst layer disposed on thesilver-containing extruded zeolite substrate, wherein the catalyst layercomprises a supported platinum group metal catalyst comprising one ormore platinum group metals and one or more inorganic oxide carriers. 2.The three-way catalyst of claim 1 wherein the zeolite is selected fromthe group consisting of a beta zeolite, a faujasite, an L-zeolite, a ZSMzeolite, an SSZ-zeolite, an AEI framework zeolite, a mordenite, achabazite, an offretite, an erionite, a clinoptilolite, a silicalite, analuminum phosphate zeolite, a mesoporous zeolite, a metal-incorporatedzeolite, and mixtures thereof.
 3. The three-way catalyst of claim 1wherein the zeolite is selected from the group consisting of betazeolite, ZSM-5, SSZ-33, Y-zeolite, and mixtures thereof.
 4. Thethree-way catalyst of claim 1 wherein the one or more platinum groupmetals is selected from the group consisting of platinum, palladium,rhodium, and mixtures thereof.
 5. The three-way catalyst of claim 1wherein the one or more platinum group metals is palladium and rhodium.6. The three-way catalyst of claim 1 wherein the one or more inorganicoxide carriers is selected from the group consisting of alumina, silica,titania, zirconia, ceria, niobia, tantalum oxides, molybdenum oxides,tungsten oxides, and mixed oxides or composite oxides thereof.
 7. Thethree-way catalyst of claim 1 wherein the extruded zeolite substrate isa flow-through substrate.
 8. The three-way catalyst of claim 1 whereinthe silver-containing extruded zeolite substrate comprises from 1 to 700g/ft³ silver.
 9. An exhaust system for internal combustion enginescomprising the three-way catalyst of claim
 1. 10. The exhaust system ofclaim 14 further comprising: a selective catalytic reduction catalystsystem; a particulate filter; a selective catalyst reduction filtersystem; a NO_(x) adsorber catalyst; or combinations thereof.